1. Field of the Invention
The present invention relates to color developing ink. In greater detail, the present invention relates to a color developing ink which can produce a colored substance upon reaction with an almost colorless organic compound.
2. Description of the Prior Art
The contact reaction between electron donating or proton accepting colorless organic compounds (hereinafter called "color formers") and electron donating or proton releasing solid acids (hereinafter called "color developers") to produce color developed images has been known for a long time. That reaction has been practically utilized in pressure-sensitive copying sheets as disclosed in, for example, U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250 and 3,672,935, heat-sensitive recording paper as disclosed in, for example, Japanese Patent Application (OPI) Nos. 4160/68 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"), 7600/68 and 14039/70, and U.S. Pat. No. 2,939,009 and so on.
Moreover, a printing process in which colored images are obtained by supplying a color former-containing ink to a color developer coated sheet is disclosed in German Patent Application (OLS) No. 1,939,962.
In conventional color developer sheets, a color developer is, in general, coated over the entire surface of the sheet and a desensitizing ink is coated utilizing a printing technique in the areas where images are not desired. As a result, these color developer sheets are very expensive. A color developing ink has been on the market for print coating a color developer only in the areas where images are wanted. However, the use of such an ink has been limited to a flexographic or a gravure printing. These inks contain an organic solvent, such as ethanol or toluol, having low boiling point and for this reason they cannot be used in conjunction with general relief or offset printing machines.
Several types of relief printing color developing inks have been recently reported in Japanese Patent Application (OPI) Nos. 68307/76, 80410/76 and 94308/76. These color developing inks comprise at least one compound selected from a group consisting of phenol resins, aromatic carboxylic acids and the metal salts thereof, isopropylnaphthalene, diphenyl methane, glycols, solvents having a boiling point of 200.degree. C. or more, pigment and the like. However, each of these inks markedly swells the rubber roll of the printing machine or evaporates to dryness on the rubber roll and, as a result, the ink cannot sufficiently exhibit its color developing ability on the surface to be printed, and light resistance of colored images produced is weak and impractical.